The present invention relates to solder connections that can be used, for example, for interconnection of microelectronic components.
The microelectronics industry has experienced an ever increasing demand for smaller and faster electronic devices able to support a greater number of increasingly complex and sophisticated functions. Accordingly, there is a continuing trend in the semiconductor industry to manufacture small, low-cost, high-performance, and low-power integrated circuits (ICs) with densely packed contact pads. However, such ICs are difficult to connect to each other or to other devices. For example, an IC having a size of 1×1 cm may have 70000 contact pads that must be connected to other components. A common way to connect contact pads to other components is by solder; solder connections are inexpensive, and advantageously they can be easily undone if rework is needed (e.g. to replace a defective component). However, densely packed contact pads can be shorted (“bridged”) by molten solder. Consequently, the solder volume must be rigidly controlled, and sometimes only minimal amounts of solder can be used, resulting in weaker connections. Therefore, solder connections are sometimes replaced by slower, more expensive diffusion bonding (which involves pressing and holding the contact pads together at a high temperature until they get attached to each other through interdiffusion). There is a need for inexpensive soldering techniques suitable for densely packed contact pads.
Another problem with solder connections for densely packed contact pads is a high planarity requirement. FIG. 1 illustrates components 110, 120 whose respective contact pads 110C and 120C are joined together by solder 130. Any one of components 110 and 120 can be a semiconductor wafer with integrated circuits (ICs), or can be a semiconductor die (also called “chip”, obtained by dicing a wafer), or can be a non-semiconductor substrate (e.g. a printed circuit board). Components 110 and 120 are shown as if perfectly planar, but perfect planarity is hardly ever achievable. FIG. 2 shows the same structure with a non-planar component 110. The distance between contact pads 110C.2 and 120C.2 is larger than between contact pads 110C.1 and 120C.1, and the distance between contact pads 110C.3 and 120C.3 is still larger. Since the planarity is not perfectly controllable, solder bumps 130 have to be sufficiently large to accommodate the worst case, such as between contact pads 110C.3 and 120C.3. However, larger solder bumps can cause bridging. For a 1×1 cm die with 70000 contact pads, the pitch P (FIG. 1) between the adjacent contact pads is about 38 μm, and for reliable solder interconnection the die planarity should be below 5 nm, which is difficult or impossible to achieve in modern technology. Therefore, improved interconnection techniques are desired.